boiler expansion types
- Thread starter BUILDER
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read the bumph 
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N
noonespecial
As kevplumb said 'read the bumph' or check with the supplier / manufacturer. Don't know about that boiler but a lot of the pellet burning boilers produced abroad are designed and tested for operation under pressure only.
You could end up with 'kettling' or cavitation in the boiler, which at best would be noisy and at worst could mean an early replacement of the heat exchanger.
You could end up with 'kettling' or cavitation in the boiler, which at best would be noisy and at worst could mean an early replacement of the heat exchanger.
N
noonespecial
Your kettle works fine without additional pressure inso far as you get hot water but you also get what has now become a term in boiler technology known as 'kettling' eg the noise when a kettle boils.
This is due to air / gas bubbles being formed as the water nearest the heat source (electric element or at the bottom in a kettle placed on a gas stove) changes state from liquid to gas (steam) which occurs at 100 degC at normal atmospheric pressure. If you seal the kettle or boiler and provide an increase of pressure inside, the boiling point of the water increases and the 'kettling' occur at a higher water temperature. The 'kettling' causes vibration (bangs) which can damage a boiler if severe.
The problem is compounded if the heat transfer surface is relatively small or the primary heat source is confined to a relatively small area and therefore localised overheating occurs. That is partly where good boiler design comes in, to ensure the heat from the primary source is transferred rapidly and evenly though the secondary heat medium (water). It is essential to ensure that adequate water flow is always achieved through boilers to maximise on the heat transfer capabilities.
Open vented boiler systems are something of a rarity on the content and therefore most of the boilers manufactured there are not designed for open vented use.
This is due to air / gas bubbles being formed as the water nearest the heat source (electric element or at the bottom in a kettle placed on a gas stove) changes state from liquid to gas (steam) which occurs at 100 degC at normal atmospheric pressure. If you seal the kettle or boiler and provide an increase of pressure inside, the boiling point of the water increases and the 'kettling' occur at a higher water temperature. The 'kettling' causes vibration (bangs) which can damage a boiler if severe.
The problem is compounded if the heat transfer surface is relatively small or the primary heat source is confined to a relatively small area and therefore localised overheating occurs. That is partly where good boiler design comes in, to ensure the heat from the primary source is transferred rapidly and evenly though the secondary heat medium (water). It is essential to ensure that adequate water flow is always achieved through boilers to maximise on the heat transfer capabilities.
Open vented boiler systems are something of a rarity on the content and therefore most of the boilers manufactured there are not designed for open vented use.
You seem to be mixing up two different phenomena.noonespecial said:
Firstly, water absorbs air but the amount of dissolved air in water increases with pressure and reduces with temperature. This is why very hot mains water emerging from a tap (and experiencing a sudden drop in pressure from perhaps 3 bar to 0 bar) will appear milky - the dissolved air is suddenly released from solution as millions of tiny bubbles. It also explains the appearance of bubbles in a kettle as the water heats up. Some air will remain in solution until boiling point is reached, when the water is said to be deaerated.
Secondly steam (gaseous water) forms at the boiling point, but this point varies with pressure, as you said. It is the localised boiling that generates kettling sounds and more serious noise, caused when the bubbles of steam collapse once they move away from the heat source. This is because steam occupies 1600 times the volume of water.
N
noonespecial
Yes you are right. I was trying to answer a fairly technical matter in as non-technical way as possible. Didn't work
Let me try!
A typical kettle is 2.4 kW to heat a couple of litres of static water.
A typical boiler is trying to put say 25 kW into a flowing two litres of water.
The heat transfer through the boiler's heat exchanger surface is about 10 times greater but still into the same volume of water.
To reduce the effect of the water boiling on the heat exchanger surface, many models are pressurised and this has the effect of increasing the boiling point of the water thus reducing "kettling".
A further point is that closed circuit system water has had all the dissolved air boiler out of it so that is taken out of the equation.
Tony
A typical kettle is 2.4 kW to heat a couple of litres of static water.
A typical boiler is trying to put say 25 kW into a flowing two litres of water.
The heat transfer through the boiler's heat exchanger surface is about 10 times greater but still into the same volume of water.
To reduce the effect of the water boiling on the heat exchanger surface, many models are pressurised and this has the effect of increasing the boiling point of the water thus reducing "kettling".
A further point is that closed circuit system water has had all the dissolved air boiler out of it so that is taken out of the equation.
Tony
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